Well, I read through all those, and it didn't really help much. There was nothing really specific about background/backdrop locations.
However, I did another web search, and found that there's another existing tool now which makes my hack redundant. "wallpapoz", which has apparently been around for some time now (since 15?). Using this resolves worrying about hacking backend files.
Most people here probably already knew this.
But I'm adding it to this thread so if I forget this the next time I blow away my install, I'll be easily able to find it from searching on my old posts.